bullyyoudaoicibaDictYouDict[bully 词源字典]
bully: [16] Bullies have undergone a sad decline in status. In the 16th century the word meant ‘sweetheart’: ‘Though she be somewhat old, it is my own sweet bully’, John Bale, Three laws 1538. But gradually the rot set in, its meaning passing through ‘fine fellow’ to ‘blusterer’ to the present-day harasser of inferiors. In the 18th and 19th centuries it also meant ‘pimp’.

It is probably a modification of Dutch boele ‘lover’ which, as a term of endearment, may have originated as baby-talk. This bully has no connection with the bully of bully beef [18], which comes from French bouilli, the past participle of bouillir ‘boil’. The bully of bully off [19], a now discontinued way of starting play in hockey, appears to come from a term for ‘scrummage’ in Eton football, but whether that is related to the cruel bully is not clear.
[bully etymology, bully origin, 英语词源]
bully (n.)youdaoicibaDictYouDict
1530s, originally "sweetheart," applied to either sex, from Dutch boel "lover; brother," probably a diminutive of Middle Dutch broeder "brother" (compare Middle High German buole "brother," source of German Buhle "lover;" see brother (n.)).

Meaning deteriorated 17c. through "fine fellow" and "blusterer" to "harasser of the weak" (1680s, from bully-ruffian, 1650s). Perhaps this was by influence of bull (n.1), but a connecting sense between "lover" and "ruffian" may be in "protector of a prostitute," which was one sense of bully (though not specifically attested until 1706). The expression meaning "worthy, jolly, admirable" (especially in 1864 U.S. slang bully for you!) is first attested 1680s, and preserves an earlier, positive sense of the word.
bully (v.)youdaoicibaDictYouDict
1710, from bully (n.). Related: Bullied; bullying.